WebRender is retained mode, not immediate mode. It will only repaint when repaint is needed, but if any repaint is needed, we will paint every pixel. There are CPU and GPU side caches for intermediate results like glyph atlases, so that doesn't necessarily mean we redo all the work every frame.

Just to give more perspective, invalidation is not free, and there are cases where invalidation itself takes longer than your frame budget. Also, in the case where you have to repaint everything or almost everything anyway, invalidation just made your problem worse since you spent cycles figuring out you can't skip any work.